1. Field of the Invention
The present invention relates to a motor driving device that is provided with a power failure detection function for detecting power failure or voltage sag of a power source.
2. Description of the Related Art
In a motor driving device for driving a machine tool, an industrial machine, a robot and the like, when power failure or voltage sag (hereinafter, the power failure and the voltage sag are collectively referred to as the “power failure”) occurs in a commercial input power source and the input power voltage decreases, normal operation cannot continue. For this reason, it is necessary to detect a power failure and switch operation of the driving device to protect the motor driving device, workpieces, or tools used in processing and the like. (See JP8-227307A, JP10-263973A and JP11-178245A.)
As a technique for detecting the power failure, for example, JP2006-14546A describes a technique in which a three-phase AC input voltage is converted into an equivalent voltage vector on two-phase coordinates and an amplitude of the vector is calculated to obtain an amplitude of the input voltage. In this technique, if the value of the input voltage amplitude is continuously less than a predetermined reference voltage value for a predetermined reference time period, it is determined that a power failure has occurred.
However, the time during which the motor driving device can continue normal operation, or power failure tolerance, varies depending on the applied power source voltage amplitude. Consequently, in the technique in which the voltage amplitude is compared with the single reference value to detect the power failure as described in JP2006-14546A, the motor driving device may not be able to be protected or, conversely, the protection may be excessive.
In order to address the above problem, JP2005-192353A proposes a technique in which a plurality of voltage drop detection sections having different reference voltage and reference time values are provided and the power failure is detected based on logical OR operation of outputs from these detection sections.
If an increased number of the voltage drop detection sections are provided according to this proposed technique, the power failure can be detected according to the power failure tolerance of motor driving device. However, a computational load on a computing section of the driving device is unpreferably increased.